A well-known type pressure sensor has one or more strain gauges mounted on a diaphragm by bonding or the like, and detects strains of the strain gauges caused by a deformation of the diaphragm when a pressure is applied.
The electrical resistance of such a strain gauge is changed by only a small amount of deformation. Typically, a strain sensor has a bridge circuit consisting of four strain gauges and detects pressure as a differential voltage output from the bridge circuit proportional to a pressure application. This bridge circuit can compensate temperature characteristics of the individual strain gauge. That is, when the four strain gauges have the same temperature characteristics, the deformations of the four strain gauges caused by a temperature change are the same, and therefore the strain sensor output is not changed.
When the fluid pressure to be measured is low, there are often used silicon film pressure sensors in which a part of a silicon substrate is thinned to form a silicon diaphragm (pressure sensing film) and one or more strain gauges are formed in the surface of the silicon diaphragm by an impurity diffusion. This type of sensor has a high sensitivity and can be integrated in an IC (integrated circuit).
However, this type of sensor is not suitable when the fluid pressure to be measured is high and/or the fluid is corrosive. For such high pressure and/or corrosive cases, there are often used pressure sensors that are formed by bonding one or more strain gauges (or a strain sensor having one or more strain gauges thereon) to a metal diaphragm.
Patent Literature 1 (JP 2005-227283 A) discloses a pressure sensor in which a lateral thermal expansion in a first diaphragm made of a first material is transferred to a strain gauge on a strain sensor made of a second material through a first bonding layer that bonds at least a part of the strain sensor and the first diaphragm. The first diaphragm made of the first material has a larger thermal expansion coefficient than the second material strain sensor formed in a first region. A lateral expansion in the first diaphragm caused by a temperature change is transferred to the strain gauge on the strain sensor through the first bonding layer that bonds at least a part of the first region and the first diaphragm.
Patent Literature 2 (JP 06(1994)-241930 A) discloses a semiconductor film pressure sensor comprising a diaphragm including: a diametrically extending thick beam; a thin diaphragm film (occupying the area other than the beam) and; a strain sensor formed on the thick beam. There are steps formed between the thick beam and the thin diaphragm film.